Corner Tub Shower Best Way To Grout Tile "corners" Of Tub/shower?

Best way to grout tile "corners" of tub/shower? - corner tub shower

My house has a little bit and the corners of my tiled shower sustained / Bath have split a few. The tile is in great shape, it is only a few corners that separate.
Is it necessary to remove the old mortar before applying again? Do I need a grout knife? I have never used the mortar, so that except for a dry, clean, there is a preparation I first? So, I'm sure that it is necessary to wait 2-3 days before bathing / display in the bathroom.
Any information or direction you can give me would be very grateful!